A vehicle's Crankshaft Position Sensor (CPS) can give non start problem particularly Fiesta/Escort (corrosion in connector)Clio/Megane's (dirty sensor).
These are normally on the clutch bell housing, but can also be on the crankshaft pulley (next to alternator drive belts). If possible pull connector apart and clean connection with WD40 (these sensors produce their electricity about 0.2-2.0 AC volts when engine turning, so can easily be lost due to corrosion) if still non start next check if sensor is working by using a volt meter.
Set meter to AC volts, you might need to attach some leads to sensor to test, loud speaker leads often are the right size for this, with meter set on Ohm's check the resistance should be around 200-500 Ohm's normally, if really high resistance try to find correct specification but normally would be faulty, if seems ok set meter to AC volts and crank engine being careful of moving parts, you should get a reading of between 0.2 and 2.0 volts with engine turning, if no reading remove sensor and clean, sensor some times are contaminated with particles from the clutch, which can stop them working.
Refit and test to see if producing AC electricity. Sometimes it can be very hard to get the senor out, a trick that will sometimes work is, if you can see a hole in the bell housing near the Crankshaft Position Sensor (CPS) sensor try spraying carburettor cleaner onto the flywheel, when cranking the engine, this will sometimes clean the sensor and the vehicle will start.
If have an EOBD scan tool with live data check for RPM, if this ok so is the CPS.